Our goal is to understand the functional organization of the labyrinth in terms of the overall operation of the vestibular system. Recent morphophysiological studies done in our laboratory have established a relation between the peripheral innervation of a vestibular afferent and its discharge characteristics. Specifically, three physiological classes of afferents were recognized: regular units, irregular high-gain units and irregular low-gain units. The first class corresponds to dimorphic (and bouton) units, the second to dimorphic units, and the third to calyx units. We are now interested in determining the function of the various classes of afferents, which can only be done by ascertaining how the brain makes use of the information provided by them. The approach is to use morphophysiological techniques in the chinchilla to trace the central trajectories of the three afferent classes and, thus, ascertain if they differ in the topography of their projections to the vestibular nuclei, other brainstem sites and the cerebellum or in the morphology of their axonal branches and terminals. Axons are impaled near their entrance into the vestibular nuclei, they are physiologically characterized, and then injected with horseradish peroxidase (HRP). After suitable histochemical processing, the central and peripheral trajectories are reconstructed. A single, physiologically characterized afferent is injected in each nerve. The experiment is repeated until a sample of approximately 20 axons from each of the three classes is obtained. The data should provide a detailed picture of the projection patterns of each class, as well as the central branching and innervation patterns of individual axons. A similar analysis will be done for all five endorgans. To provide a context for our morphophysiological data, anatomical tracer studies will be done on the commissural, vestibulo-ocular, vestibulospinal and vestibulocerebellar pathways in the chinchilla. The results should provide insights into the functional organization of the labyrinthine receptors and of their central targets and into the evolution of the vertebrate labyrinth. In this last respect, the calyx unit and the Type I hair cells in innervates are of particular interest. These structures are recent phylogenetic acquisitions, first making their appearance in reptiles and then becoming distinctive features of the labyrinth of birds and mammals. An understanding of the function of the calyx unit should shed light on its evolutionary significance.